[time-nuts] schematics of frequency counter
lllaaa at gmail.com
Mon Jan 5 00:39:23 EST 2015
TPS79333 is quite cheap and adm7150 is 60 times the price of it. These 2
ldo has different package, it not easy to replace it directly. I have tried
power the system with 18650 battery, the performace is almost the same with
5v from laptop USB port. So I think psrr might not be a issue.
If the noise generated by LDO is the problem, can I filter it out by
puting more caps?
Neil Schroeder <gigneil at gmail.com>于2015年1月3日星期六写道：
> I would reconsider the LDOs while you have some time to play with them.
> The TPS79333DBVR is not even remotely "ultralow noise" at most offsets,
> despite what TI may say.
> For 5V5 and under up to about 600ma, I would suggest you take a look at
> ADM7155 (adjustable) or ADM7154 (fixed). If you need 800ma, the 7150 is
> here for you.
> The TI part has nearly 35 uVRMS of noise at just 2.8a with not very good
> PSRR. The 7155 will produce less than 1.0 μVRMS Total Integrated Noise
> from 100 Hz to 100 KHz and 1.6 from 10 to 100 KHz,and that's without an RC
> noise compensation network. Its a fabulous part for a sensitive
> application like yours.
> I'm hoping I am right in assuming that the noise performance of the LDOs
> a concern, and don't mean to even dare suggest that you've done anything
> but a great job! :-)
> On Sat, Dec 27, 2014 at 5:34 AM, Li Ang <lllaaa at gmail.com> wrote:
>> Hi Charles,
>> In my circuit, the VCC is 5v. I've noticed my bias and emitter
>> is something need to be changed. I will play with the resistors and see
>> it improves. Thanks.
>> 2014-12-27 6:42 GMT+08:00 Charles Steinmetz <csteinmetz at yandex.com>:
>> > Li Ang wrote:
>> > RF pnp transistor is harder to get. I would like the front end works
>> >> at 300MHz.
>> >> My questions:
>> >> 1) why the difference of DC bias of the 2 NPN matters? I thought only
>> >> frequency part is useful to a counter, amplitude information is
>> >> right?
>> > You want the circuit to switch near the mid-point of the input sine
>> > and at exactly the same place every time. How you bias the transistors
>> > determines how well this is accomplished.
>> > You also want the output to switch fast and cleanly between a low
>> > very near 0v ("ground") to a high voltage very near 3v (Vcc, logic
>> > An NPN cannot do that, biased the way that you have them connected (the
>> > emitter of the output transistor Q301 can only pull the output down to
>> > little less than 1v due to R315, which may sort of work but is not a
>> > way to run 3v logic). This operation also saturates Q301, which is bad
>> > performance. See simulated results below.
>> > In order for an NPN to provide a useful output for 3v logic, (i) its
>> > emitter must be grounded, and (ii) it must either be run into
>> > use a Baker clamp. Running the transistor into saturation must be
>> > particularly if you want to reach 300MHz, and a Baker clamp raises the
>> > "logic low" output voltage to >0.5v (not a good thing with 3v logic).
>> > it is very much better to use a PNP differential pair. For a 300MHz
>> > circuit, I would use BFT93 (and even that barely gets you to 300MHz).
>> > 2) what's is the C4 in your circuit for?
>> > C4 makes Q1 and Q2 a differential (emitter-coupled) pair at RF
>> > frequencies, but not at DC. So, the circuit has no gain at DC and
>> > therefore the DC errors between Q1 and Q2 cause much less output error
>> > they would if the emitters were connected directly together.
>> > 3) If the noise is more important than the gain, what kind of
>> >> should I choose? The Ft near 300MHz ones(BFS17, 2SC9018) or Ft far
>> >> 300MHz ones(BFP420, BFP183,BFR93) ?
>> > Far beyond. The Ft is the frequency where a transistor completely runs
>> > out of gain. You want to operate at a much lower frequency where the
>> > transistor still has substantial gain, particularly with fast RF
>> > transistors, which generally have much lower DC hfe than
>> > transistors like 3904 and 3906. Note that the simulation of the
>> > you published (simulated results below) barely works at even 20MHz.
>> > noted above, even the BFT93 barely gets you to 300MHz with a 1Vrms
>> > Best regards,
>> > Charles
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